The biogeography and functional understanding of buttress roots in tropical forests

Buttress roots are iconic features of tropical rainforests, yet the mechanisms underlying their global distribution remain poorly understood. Two prevailing hypotheses, mechanistic support and nutrient acquisition, have been explored through numerous studies, but no global consensus has emerged. To address this gap, we conducted a global biogeographic analysis, identifying 847 tree species capable of generating buttress roots and integrating this data with forest inventory datasets worldwide. Our findings reveal that while buttress root species are associated with higher canopy heights, they are not adapted to high wind speeds or shallow soils, challenging the mechanistic support hypothesis. Instead, we found that soil phosphorus limitations and relatively acidic soils enhance the presence of buttress roots, supporting their role in nutrient acquisition. Climatic factors,  including the lowest temperature of the coldest month and site water balance, also appear to constrain the distribution of buttress root species, potnetially due to photosynthesis-related trait limitations. Our geospatial models estimate that buttress root species cover approximately 21% of global tropical forests, emphasizing their ecological significance in tropical forest structure. These findings highlight the critical role of buttress roots in forest ecosystems and their importance for global conservation efforts.